Diffusers and belly pans

[JMortensen]

So ZR8ED, DOES IT WORK???

[Owen]

I been waitin for this to take shape, great work! You may have explained this before, but what does the leading edge (by the diff) look like? Is it flat or is it angled to direct air trapped around the rear of the diff out to the diffuser?

 

Oh wait, if it was angled that way, you'd have 7-9 degrees in the opposite direction... I guess it should just be flat? What about air trapped around the gas tank area above the diffuser? Am I being paranoid???

Owen

[johnc]

The air on top of the diffuser creates the downforce as it tries to move through the top of the diffuser to get the low pressure area. Air in the fuel tank area is necessary to make the diffuser work.

 

I'm concerned with the pan underneath the diffuser in this application. The diffuser should be open to the ground. There's a chance that ground level air will be at a higher pressure then the air flowing through the diffuser which would cause pressure on the bottom side of the pan, negating the downforce.

[ZR8ED]

Owen: The rear pan ends at the diff mount. There is no edge to keep air from going over the top of the pan, around the gas tank and muffler area, and out through the mesh at the back.

 

John: I'm not too sure what what you mean. The pan is very similar to my front belly pan. Any air that goes above the pan will travel slower than the air under it. That is the definition of down force is it not? Air planes use the reverse of this to gain lift.

 

The air that is going over the pan will exit through the mesh. This is probably why the 280's had holes cut out in the rear valence, to let trapped air out from around the gas tank area. Same with my vents on my wheel wells. To let air trapped in the wheel well area, so they don't act like parachutes. hehehe.

 

As far as does it work? I have not had the car beyond 200kph, so I doubt, any effect will be noticeable under that speed. It looks pretty mean though..hehe not too scientific...

 

I'm getting a bit confused now talking about the pan and the diffuser as two different items? I was thinking that the pan was supposed to channel the air that is already under the car and makes the air move faster under the car, than over it thus creating down force. The principal of this design seems sound, and this pan design has been partially taken from a very successful GT1 240 that my friend had built years ago. The original had used zues fasteners, and was not as nice looking, but is pretty much the same. Though my car will never likely see anything beyond 160mph like the race car does.

 

As the summer progresses, I hope to get some seat of the pants info. Hopefully a fall track day will come up, and I can get some more experience with it. This is mainly a street car, so in the end, its more about the form than the function... as long as it doesn't make the car handle worse, I'm ok with it. It sure looks better than the ugly off side tank hanging down.

 

Scott.

[johnc]

The air that is going over the pan will exit through the mesh.

 

Never mind, I was confused by the pictures. I thought that the air going through the diffuser was exiting through the mesh.

[Owen]

OK, I understand now. So what about those diffusers that are built into the bumpers like on the rice cars, just extra weight?

here's one from an RX7 that would look functional

 

This one is molded to the shape of the rear of the car, don't see any exists for air at the top tho...

 

 

But I don't know about this one....

or this

 

Owen

[ZROSSA]

Owen,

Strictly speeking the diffuser should work on a totally sealed under pan so that no air can get above it. Look at a 360 ferrari or any high power single seater from the eighties onward. I have podered the diffuser issiue for a long time now for my corrola race car. My rules say that I cant have a diffuser as such and we have a ride height restriction of 3 inchs. I have come to the conclusion that the best I can do is clean up the flow in the area and make sure there are no restrictions to the air flow coming out of the back of the car. If you dont have end plate the the low pressure developed by the upturn will suck in air from the sides making the diffuser redundant. I have used a similar aproach on my Z. I was not wiling to try to smooth out the chassis so I have just made my rear bumper tuck under the rear and will attach it to the fuel tank with some super strong tape or something like that. I had to cut away some of the rear valeance (sp?) to make this work. Kind of looks like a diffuser but will never really function like one.

 

Douglas

[Mikelly]

I hope new members are reading and learning... This stuff is EXACLTY the stuff HybridZ is made of...

 

Zrated/ Scott... EXCELLENT work on that diffuser. I'd be interested in the functionality. If nothing else, it fits the theme, and you are working with front and rear pans to gain, lose some aero... The key is to slot, and then add pivots so that you can at least adjust them.

 

I had posted some pics of the C5 because it uses a LOT of aero tricks to get the CD numbers down to a low of 29. That car is extremely flat from the underside, and I'll have to find those pics and re-post.

 

I'm not so sure a rear difuser is really that necessary, but Scott's looks SWEET! The units built into the body work have always looked of the CHEEZE WHIZ/ Ricaroni era to me, but YMMV!

 

Muslaines corner... that is a VALUABLE website, One the new members should bookmark and add to their lesson plans!

 

Mike

[Guest binchillin]

I think trying to manipulate the airflow under the car to gain a benefit is asking too much from a road car. I want to figure out how to stop the airflow from manipulating me! Instead of trying to get the air to stick(downforce?) to the bottom of my car, maybe there is more to gain from controlling the flow to NOT stick to the bottom of my car. After all, the air is twirling around everywhere already.

To get a downforce means the air is acting on a surface, could be more pressure or less pressure. You still need to un-stick the air, if you don't you have drag.

Why worry about manipulating the airflow? Why not worry about making sure it just slides over or under w/o influencing anything? Like a golf ball.

Isn't there a gain to be had from being free of "ground effects"?

What about getting a car to go through the air free of drag. Just gravity holds it to the road. Think about it. The wings on some cars don't add downforce to help in cornering, the wing is there to keep the car from going airborn! The top surface of a Corvette going 120MPH is LIFTING the car, so it seems to me, the wing is just to help the car not fly!

For a road car, I think people smarter than me should think about going through the air at 100MPH w/o any ground effects. So if a car drove past me while I was walking on the sidewalk, I wouldn't feel any pressure wave like I feel today.

So I think, for a road car, you want to figure a way to make air not stick AT ALL to any part of the under carriage. If you go after downforce, you get drag too. That's OK for a race car(even though I think air foils hurt racing more than helping) but I think there is a gain from my theory.

I saw a tv show about the first race cars that had a wing( white chapparell?, sorry about the spelling) and everyone said he's just making more drag.....ity won't work. But that guy made CONTROLLABLE drag and used it to his benefit. Has anybody thought about keeping the airflow from sticking to the car? If you know how to control the concept, there's a benefit!

[Z-TARD]

It's not so much that the air is sticking to the surface of the car, but that it is being displaced by the surface of the car that creates the most drag. To eliminate drag and make it so that no pressure wave would be felt if a car passed you, it would have to be shaped like an arrow. A lot of land speed record cars are shaped like that, about 25 or 30 feet long, but only 2 feet wide. This would be a totally impractical design for any car that intends to go in a direction other than long straight lines. Automotive areodynamics is always a compromise of drag vs downforce, but there are ways around that. Using the air under the car to generate downforce is like getting downforce for free, without the associated drag penalty. The air under the car is going to create drag no matter what you do, so if you can clean up the airflow a little and generate downforce in the process, you are already a few steps ahead of the guy that just put a giant wing on his car and called it good. Using the diffuser/bellypan to create downforce will let you use a smaller wing /spoiler in the back to generate the same amount of total downforce as a much larger wing, without the drag the larger wing would create as well. Hope this helps explain what they are trying to accomplish here.

 

ZR8ED, let us know how how it works when you get a chance to open it up a little

 

Mike

[Guest binchillin]

You could build a car in the shape of an arrow, and you're right it's impractical, but the arrow is only looking for stability. and so is the rocket car, and so does the Corvette with a rear wing. The wing isn't for downforce, it's for stability(so the ass-end doesn't get too light, right?).

What are you guys trying to do? Are the airfoils meant to produce downforce? Or are you looking to unstick the car from the airstream(isn't a diffuser doing that also?)

Your example about the arrow I understand. But a sphere would go through the air pretty well too and wouldn't resist a change in direction like an arrow.

I also don't believe you can get downforce for free. I'm just saying forget about touching the wind for a road car, get away from it by going through the air like a golf ball. It's not very romantic, but that's not what this forum is about, right?

Stop being a wind whore, it only gets you into trouble. I'm going to go watch the wind whores at the Nurburgring now. Peace!

[Mikelly]

Binchillin', Most of these guys are building cars capable of reaching near 200 MPH in the right conditions. Will they ever drive the car at that level? Only they can answer that. But to tell them to "Stop Chasin" or "Stop being" is NOT what this site is about.

 

If you have something useful to contribute, then your imput is welcome. If not, Have fun watching the "Wind Whores at the 'Ring". But do yourself a favor and read the rules posted at the top of this category.

 

Mike

[Drax240z]

Isn't there a gain to be had from being free of "ground effects"?

 

What about getting a car to go through the air free of drag. Just gravity holds it to the road. Think about it. The wings on some cars don't add downforce to help in cornering' date=' the wing is there to keep the car from going airborn! The top surface of a Corvette going 120MPH is LIFTING the car, so it seems to me, the wing is just to help the car not fly!

 

For a road car, I think people smarter than me should think about going through the air at 100MPH w/o any ground effects. So if a car drove past me while I was walking on the sidewalk, I wouldn't feel any pressure wave like I feel today.

 

So I think, for a road car, you want to figure a way to make air not stick AT ALL to any part of the under carriage. If you go after downforce, you get drag too. That's OK for a race car(even though I think air foils hurt racing more than helping) but I think there is a gain from my theory.

 

Has anybody thought about keeping the airflow from sticking to the car? If you know how to control the concept, there's a benefit![/quote']

 

Wow, what was Bernoulli thinking? Your theory makes areo design so much easier than the laws of physics do!

 

First off, you CANNOT make a car with zero drag. Even if you are to lessen downforce to a point where there is no lift, and no downforce you still have huge amounts of drag. Drive down a highway at 60mph with your hand out of the window. Find an angle in which your hand isn't forced up or down, and tell me if you've eliminated drag.

 

There are methods to reducing drag, as Mikelly says the corvette has a very slippery cd of .29. In order to reduce this to zero you need a medium with zero viscosity (a vacuum) or a cross sectional area of zero.

 

"Just the facts ma'am."

[DaleMX]

This is a very interesting thread that really shows the tech ability of the folks at Hybridz. I have learned more here in the last 6 months than anywhere else that I can remember. I'm glad you guy's are here to share your knowledge and experience. I just hope I can effectively utilize it to my advantage and possibly share some of mine.

 

Thanks all,

Dale

[Drftn280zxt]

Well to achieve more downforce, you need the air underneath the car to move faster. Lets look at an airplane wing. The underside is flat and the top is curved. Okay now a little physics stuff. You have air moving "X" miles per hour or whatever before the wing. As it goes around the wide it wants to keep up with itself (eliminating vacuums). In order to fo this the air going over the top of wing must move farther over the same amount of time that the air underneath does at the same time. Thus the air over the top moves faster creating a low pressure area. (FYI, if a meteorologist says a low pressure "zone" is moving into your area, its gonna be windy). Okay now the slower moving area underneath the wing creates high pressure. The high pressure wants to move to the low pressure. Thus creating lift. Okay now on a car it would be upside down. Yes you want the car to be as flat as possible (less drag) but you also wanting to creating the most distance possible. You want the air to move faster underneath the car than on top. Thus creating downforce. Diffusers to my understanding merely just tell the air where to go, the bellypans are what deliver the much needed low pressure. But then again there is another approach. Simply make the bottom of the car as flat as possible and but in diffusers to make the underside air less turbuleant (sp?)(less turbuleant, less drag). Then put wing/spoilers(inverted airplane wing) on top to do your downforce adjustment. That is mathematical terms you are working a one variable equation versus two. If you can simply do fine aero adjustments on top only, things can be much easier. At least that how GT cars and other race cars do it.

 

To my understanding most "Rice" wings are designed incorrectly with the exception of the carbon fiber GT wings with the rised portion in the middle. The top surface should be flat with minimal angle (drag causing) and have a longer area underneath to create low pressure.

[blueovalz]

Interesting subject in light of a discussion I heard the other day in reference to a new look into why a wing creates lift. I was unable to catch the entire discussion, but in essence, the Bernoulli (spelling?) principal is being questioned, and some researchers claim that the air flow instead follows the top surface of the wing, and in doing so, leaves the wing in a downward direction. This mass of air, accelerated in a different direction (downward) causes an opposite reaction of "lifting" the wing. This discussing immediately brought to mind visions of the F1 wings and the "rooster tail" created in the rain. The water (air) is immediately forced upward as it exits the car, and if this discussion is correct, the equal and opposite reaction to this is to force the car downward. You can make a barn door fly with enough speed, and this would also complement that analogy as well. I know when I was involved in R/C aircraft, I experimented with various airfoils, and the curved top/flat bottom was not the only design that created lift. A symetrical cord worked as well. When you watch a 727 wing double in width due to slats and flaps, and see what the wing's cross section looks like in this configuration, it's not hard to be swayed by this discussion. The researcher's arguments appeared to have merit. Even a flat bladed fan creates "lift" with the proper angle as it spins, so...

 

Perhaps in 20 years we will decide that it will be taught in school that it's a combination of both instead of one or the other, kind of like the wave verses the particle theory of light.

[Drax240z]

Actually Terry you bring up a good point, I have read some of the counter-Bernoulli theories as well, and they make some good points as you discuss. Basically the arguement is that by forcing the air in a direction, and equal and opposite force is exerted on the 'wing', be it a barn door or whatever... Hence the ability to fly. I think the biggest stumbling block for me with Bernoulli is this:

 

Take a wing for example, you have air seperating at the leading edge of the wing, and following different paths to meet at the end of the wing. Due to the wing being curved the air over (or under) the wing much be faster than on the other side. This velocity differential creates a pressure differential which in turn creates lift.

 

What I wonder is... why must the air travelling on both sides of the wing travel the span of the wing in the same amount of time?

 

Anyway my comment in the above post wasn't intended to start this discussion.

[blueovalz]

The latest AW has a photo of the new Ferarri 430. The photo is a rear shot showing a diffuser, but this is an odd design in that my first impression is "rice". The reason I say this is that this externally viewed part follows the bottom of the car up and around the rear at a quite high angle (far beyond normally accepted angles and very short). I can't imagine the entire part of this to be functional, but at the same time, I can't imagine Ferrari sticking something on their car that is not "functional" through and through. Here it is:

 

[ZROSSA]

Terry,

Older f1 cars had similar diffusers. I think it works becuase the area in front of the rear wheels is wider. The diffuser can run a higher angle becuase as the air goes between the rear wheels the width of the diffuser narrows so it must rise to maintain volume. It is likely that this is a better diffuser design then what we see on the race cars these days as the production car doesnt have the rule constrants that most race cars have these days.

 

Douglas

[Drax240z]

Well the problem is that flow seperation occurs regardless of velocity, and pretty much a fixed angle for air. (~10-12*) One large benefit of a gradual curve on a diffuser is that you have no sharp angles and you give the air a chance to 'turn' without seperation. If you have a gradual enough curve then you can essentially turn a very large angle without flow seperation. (ie: the front of a 747)

 

What I suspect with the Ferrari in question is that they did some serious wind tunnel testing and found out that they could follow the body lines much more nicely, and still create significant downforce, even with localized turbulant airflow. I am sure it is very functional on that car, but I would bet that there were some compromises made in terms of looks in this case.